Structural diversity and carbon sequestration of forest stands. Tradeoff as modified by silvicultural thinning

Abstract

Abstract In forest management, merging stand structure diversity with carbon storage is essential for resilience and climate mitigation. This study assesses (i) how structure diversity in stands of spruce (Picea abies (L.) H. Karst.), pine (Pinus sylvestris L.), beech (Fagus sylvatica L.), and oak (Quercus robur L. and Quercus petraea (Matt.) Liebl.) varies with age, site quality, and applied thinning grade; (ii) these factors' impact on carbon stock; and (iii) the link between structure diversity and carbon stock. Analyzing 26 long-term thinning experiments, we used the Gini coefficient of tree heights to measure structural diversity and species-specific biomass functions for carbon stock assessments. Our results show structure diversity, highest in beech and spruce, decreases with stand age and on richer sites. Thinning enhances structure diversity in spruce and beech but reduces it in pine and oak. Unthinned or only moderately thinned mature spruce and beech stands outperform pine and oak in carbon stock (200–300 vs. 100–150 MG C ha-1). C- and D&E-grade thinning halves carbon stock. A decrease in vertical layering with increased carbon storage varies across species. Given the same carbon stock, thinning from above maintains structure diversity in spruce and beech, while non-thinning or thinning from below promotes structure diversity in pine and oak. We argue that reduced thinning in previously D&E-grade thinned stands may reduce structural diversity but improve carbon storage (+ 100–200 MG C ha− 1) of the four considered species in the next 3–5 decades. We discussed options for reconciling structural diversity and carbon storage by silvicultural management.